This invention relates to projection optical devices for use with reflective liquid crystal displays, e.g., LCoSs. More particularly, the invention relates to an arrangement of polarization components designed to work with reflective liquid crystal displays and maintain an effective separation of illuminating light and reflective light.
Reflective liquid crystal displays (e.g., LCoS displaysxe2x80x94Liquid Crystal on Silicon displays) work with polarized light in accordance with the following: xe2x80x9coffxe2x80x9d pixels of the display reflect light without changing its polarization state and xe2x80x9conxe2x80x9d pixels rotate the polarization of the illuminating light by 90 degrees. (Alternatively, xe2x80x9conxe2x80x9d pixels can leave the polarization unchanged and xe2x80x9coffxe2x80x9d pixels can rotate the polarization, but the xe2x80x9coffxe2x80x9d equal no change in polarization and xe2x80x9conxe2x80x9d equal a change in polarization is, in practice, the most common approach.) A known optical layout of polarization components is shown in FIG. 1. See Miyatake, U.S. Pat. No. 5,327,270, entitled xe2x80x9cPolarizing Beam Splitter Apparatus and Light Valve Image Projection Systemxe2x80x9d which issued on Jul. 5, 1994, the content of which is incorporated herein by reference.
In the layout of FIG. 1, illuminating light 8 passes through an initial polarizer 9 (S polarizer) and is reflected from the diagonal of a polarization beam splitter 3 (PBS). This light illuminates LCoS 11. Light reflected from xe2x80x9coffxe2x80x9d pixels has the same polarization as the incident light and is reflected from the PBS diagonal back into the illumination system. Light reflected from xe2x80x9conxe2x80x9d pixels has a polarization orthogonal to the polarization of incoming light. That light is transmitting through the PBS diagonal and enters into the projection lens of the system (see light 17 in FIG. 1). It should be noted that in this layout, illuminating light 8 and imaging light 17 can be reversed, with the illuminating beam passing through the diagonal of the PBS and the imaging beam reflecting from the diagonal.
The PBS diagonal has a multi-layer structure, which is designed to reflect S polarization and to transmit P polarization. The typical reflection factor for P polarization is xcx9c12%, which reduces the contrast of the system. The initial polarizer 9 in front of the PBS 3 absorbs light with P polarization to maintain the contrast at the required level.
The cone of light from the illuminator typically has an F-number equal to 2.8, which creates a significant depolarization effect for skew rays. This geometrical effect is described in the Miyatake patent referred to above and requires a quarter wave plate 5 between the PBS 3 and the reflective liquid crystal display 11 (e.g., LCoS) to maintain high contrast.
The main drawbacks of this polarization layout are the high cost of the PBS and the need to use a quarter wave plate, which requires a very precise angular alignment in order to reduce the skew ray problem. Also, quarter wave plates are very sensitive to temperature which creates non-uniformities in contrast across the imager.
To address the foregoing deficiencies in the art, the invention provides an optical system (polarization arrangement) comprising:
(a) a light modulating panel;
(b) a projection lens for forming an image of the light modulating panel, said projection lens having a depth of focus D in imager space; and
(c) a tilted polarization splitter located between the light modulating panel and the projection lens, said polarization splitter comprising a substrate having a thickness T such that the astigmatism introduced by the substrate at its tilted angle is less than or equal to D.
In addition to the foregoing, during use, the optical system will also include an illumination system, with the tilted polarization splitter being located between the illumination system and the light modulating panel.
Additional features and advantages of the invention are set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein.
It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed.
The accompanying drawings, which are incorporated in and constitute part of the specification, illustrate various aspects of the invention, and together with the description, serve to explain the principles of the invention. It is to be understood, of course, that both the drawings and the description are explanatory only and are not restrictive of the invention.